Most straw bale homes aren’t straw bale homes. A straw bale home would technically be a structural straw bale home, meaning that the bales support the structural loads such as the weight of the roof and walls, wind loads and seismic forces.

Most straw bale homes are post and beam or wood-framed structures which use straw bales as insulation. As interior and exterior wall covering materials they typically use plaster, either natural plasters such as earth or lime or synthetics such as stucco-cement. Time has shown that the natural plasters last better, look better and are easier to maintain.

If they are to be healthy places in which to live, they will have certain design/building features and components which differ from conventional homes, the purposes of which will mostly be to keep moisture out of the straw. A short list includes…

NO FLAT ROOFS
Generous roof overhangs
No roof parapets
Quality roofing material, installation and maintenance
Generous clearance from grade to lower wall terminations
Proper sealing and termination techniques at wall penetrations and terminations
No plumbing pipes inside walls with straw.
The first course of bales is raised above the floor
A capillary break exists between the bottom of first course bales and the surface upon which they bear.
Permanent moisture sensors may be installed in vulnerable areas

CONCERNS ABOUT STRAW BALES

Even though moisture intrusion of the straw bales can be a serious problem, because of their sheer mass, straw bales walls are an excellent hygric buffer. This means that they're able to absorb and store a large amount of moisture without damaging effects. Exactly how much moisture is something most homeowners would rather not find out.

Although straw bale building technology has made huge strides in the last 20 years, inspectors will be asked to inspect homes which were built before good practices were understood, and although many were built in the 1970's, homes are still being built today by those not taking the time to do their homework.

Poorly built straw bale homes can be very unhealthy places to live, may generate a higher percentage of lawsuits than conventional homes and inspectors should be very specific in their contract disclaimers. Recommending testing by a qualified indoor air specialist is a good idea if an inspector sees any indication at all that things are not right. Think MOLD. Leaky building envelope designs, poor materials or inadequate maintenance can severely damage or destroy these homes.

Straw bales homes built to modern standards are very fire-resistant. Tightly-packed straw with two inches of plaster on the interior and exterior with little oxygen present inside the wall can provide a 2-hour firewall. Most straw bale homes lost to fire are lost during construction.

Left, expanding foam glues bales together, temporary corner boards help keep bales aligned. Center, this slip (first) coat has been applied with a spray gun. Right, This slip coat was applied by hand. The farther into the straw the slip coat saturates, the better the adhesion. This adhesion alone will support a 1 1/4" to 1 3/4" thick coat of plaster. Newer homes tend to use earthen plasters rather than stucco-cement at the exterior.

Left,Studs are covered with expanded metal lath to prevent thermal bridging. Center and right are the same area, before and after (temporary scaffolding is visible in the center photo).

Left, windows are wrapped in expanded metal lath to give the edges a rounded look, Center, Lathe is given a scratch coat of gypsum plaster. Right, Afer a coat of earthen plaster, the wall gets a coat of gypcrete and is finished with a colored lime wash.

INSPECTION CONSIDERATIONS

Those wanting to learn more about inspecting straw bale homes can read Straw Bale Home Basics, which includes more photos, information on construction, techniques for inspection and information on diagnosing cracks and other problems.

All straw bale photos by Kenton Shepard

Sources for More Information

Peak to Prairie Home Inspection Service

Straw Bale Homes